When not all the counter service restaurants are open, they tend to rotate them (so one day it might be Hakuna Matata thats open, the next it might be Fuente del'Oro, or Colonel Hathis etc etc).
The only way to know for sure is to pass the restaurant around or after 11.30am, as thats when the counter service restaurants tend to open for lunch. If theres a menu posted on the board outside the restaurant, its open that day. If theres no menu, the poster on the board will tell you the restaurant is closed, and sometimes theres a list on the poster of cs restaurants which are open instead.
eta - don't wait much after 5pm to have dinner in the counter service restaurants either - a lot of them close at this time or shortly afterwards, regardless of how late the park is open until, out of high summer season. We got caught a couple of nights last June, trying to get something to eat in the park at around 8pm (park closed at 11pm) and ended up in the Cafe Hyperion because the cs restaurants in Fantasyland and Frontierland were all closed by then.